Stable, color-tunable 2D SCN-based perovskites: revealing the critical influence of an asymmetric pseudo-halide on constituent ions

Orientation of Thiocyanate Ions Tuning the Electron-Phonon Interactions in Pseudohalide Perovskites

Although the importance of electron-phonon interactions on the optoelectronic properties of perovskites has been well documented, the structural origin of electron-phonon interactions remains largely unexplored. In this study, using pseudohalide perovskites Cs2Pb(SCN)2I2(1-x)Br2x as a model, we have revealed how the orientation of SCN- anions tunes the electron-phonon interactions and the effective charge-carrier mobility by utilizing femtosecond sum frequency generation vibrational spectroscopy, supplemented by photoluminescence spectroscopy and femtosecond optical-pump terahertz-probe spectroscopy. The coupling between neighboring SCN- anions decreases as the Br content (x) increases but does not have a significant effect on the electron-phonon interactions. In contrast, the orientation angle of SCN- anions has a strong correlation with the electron-phonon interaction and effective charge-carrier mobility, that is, a more parallel orientation of SCN- anions leads to a higher electron-phonon interaction and lower effective charge-carrier mobility. This finding provides a molecule-level understanding of the inorganic lattice structure in tuning electron-phonon interactions and may offer valuable guidance for optimizing the optoelectronic properties of perovskites.

X-Site Substituted 2D Cs2 Pb(SCN)2 Br2 Perovskites for X-Ray Detection

2D Ruddlesden-Popper (RP) perovskites have been intensively investigated due to their superior stability and outstanding optoelectrical properties. However, investigations on 2D RP perovskites are mainly focused on A-site substituted perovskites and few reports are on X-site substituted perovskites especially in X-ray detection field. Here, X-site substituted 2D RP perovskite Cs2 Pb(SCN)2 Br2 polycrystalline wafers are prepared and systematically studied for X-ray detection. The obtained wafers show a large resistivity of 2.0 × 1010 Ω cm, a high ion activation energy of 0.75 eV, a small current drift of 2.39 × 10-6 nA cm-1 s-1 V-1 , and charge carrier mobility-lifetime product under X-ray as high as 1.29 × 10-4 cm2 V-1 . These merits enable Cs2 Pb(SCN)2 Br2 wafer detectors with a sensitivity of 216.3 µC Gyair -1 cm-2 , a limit of detection of 42.4 nGyair s-1 , and good imaging ability with high spatial resolution of 1.08 lp mm-1 . In addition, Cs2 Pb(SCN)2 Br2 wafer detectors demonstrate excellent operational stability under high working field up to 2100 V cm-1 after continuous X-ray irradiation with a total dose of 45.2 Gyair . The promising features such as short octahedral spacing and weak ion migration will open up a new perspective and opportunity for SCN-based 2D perovskites in X-ray detection.

Inorganic-Cation Pseudohalide 2D Cs2 Pb(SCN)2 Br2 Perovskite Single Crystal

Most of the reported 2D Ruddlesden-Popper (RP) lead halide perovskites with the general formula of A_{n +1} B_n X_{3 n +1} (n = 1, 2, …) comprise layered perovskites separated by A-site-substituted organic spacers. To date, only a small number of X-site-substituted RP perovskites have been reported. Herein, the first inorganic-cation pseudohalide 2D phase perovskite single crystal, Cs₂ Pb(SCN)₂ Br₂ , is reported. It is synthesized by the antisolvent vapor-assisted crystallization (AVC) method at room temperature. It exhibits a standard single-layer (n = 1) Ruddlesden-Popper structure described in space group of Pmmn (#59) and has a small separation (d = 1.69 Å) between the perovskite layers. The SCN^- anions are found to bend the 2D Pb(SCN)₂ Br₂ framework slightly into a kite-shaped octahedron, limiting the formation of a quasi-2D perovskite structure (n > 1). This 2D single crystal exhibits a reversible first-order phase transformation to 3D CsPbBr₃ (Pm3m #221) at 450 K. It has a low exciton binding energy of 160 meV-one of the lowest for 2D perovskites (n = 1). A Cs₂ Pb(SCN)₂ Br₂ -single-crystal photodetector is demonstrated with respectable responsivity of 8.46 mA W^-1 and detectivity of ≈1.2 × 10¹⁰ Jones at a low bias voltage of 0.5 V.

Sensing Mechanism of H2O, NH3, and O2 on the Stability-Improved Cs2Pb(SCN)2Br2 Surface: A Quantum Dynamics Investigation

Although the perovskite sensing materials have shown high sensitivity and ideal selectivity toward neutral, oxidative, or reductive gases, their structural instability hampers the practical application. To exploit perovskite-based gas-sensing materials with improved stability and decent sensitivity, three adsorption complexes of H2O, NH3, and O2 on the Cs2Pb(SCN)2Br2 surface are built by doping Br- and Cs+ in the parent (CH3NH3)2Pb(SCN)2I2 structure and submitted to quantum dynamics simulations. Changes in the semiconductor material geometric structures during these dynamic processes are analyzed and adsorption ability and charge transfer between Cs2Pb(SCN)2Br2 and the gas molecules are explored so as to further establish a correlation between the geometrical structure variations and the charge transfer. By comparing with the previous CH3NH3PbI3 and (CH3NH3)2Pb(SCN)2I2 adsorption systems, we propose the key factors that enhance the stability of perovskite structures in different atmospheres. The current work is expected to provide clues for developing innovative perovskite sensing materials or for constructing reasonable sensing mechanisms.

Comprehensive Non-volatile Photo-programming Transistor Memory via a Dual-Functional Perovskite-Based Floating Gate

Photonic transistor memory has received increasing attention as next-generation optoelectronic devices for light fidelity (Li-Fi) application due to the attractive advantages of ultra-speed, high security, and low power consumption. However, most transistor-type photonic memories developed to date still rely on electrical bias for operation, imposing certain limits on data transmission efficiency and energy consumption. In this study, the dual manipulation of "photo-writing" and "photo-erasing" of a novel photonic transistor memory is successfully realized by cleverly utilizing the complementary light absorption between the photoactive material, n-type BPE-PTCDI, in the active channel and the hybrid floating gate, CH₃NH₃PbBr₃/poly(2-vinylpyridine). The fabricated device not only can be operated under the full spectrum but also shows stable switching cycles of photo-writing (PW)-reading (R)-photo-erasing (PE)-reading (R) (PW-R-PE-R) with a high memory ratio of ∼10⁴, and the memory characteristics possess a stable long-term retention of >10⁴ s. Notably, photo-erasing only requires 1 s light illumination. Due to the fully optical functionality, the rigid gate electrode is removed and a novel two-terminal flexible photonic memory is fabricated. The device not only exhibits stable electrical performance after 1000 bending cycles but also manifests a multilevel functional behavior, demonstrating a promising potential for the future development of photoactive electronic devices.

Structure and Optical Properties of Layered Perovskite (MA)2PbI2-xBrx(SCN)2 (0 ≤ x < 1.6)

The layered perovskite (MA)₂PbI₂(SCN)₂ (MA = CH₃NH₃⁺) is a member of an emerging series of compounds derived from hybrid organic-inorganic perovskites. Here, we successfully synthesized (MA)₂PbI_2-xBr_x(SCN)₂ (0 ≤ x < 1.6) by using a solid-state reaction. Despite smaller bromide substitution for iodine, 1% linear expansion along the a axis was observed at x ∼ 0.4 due to a change of the orientation of the SCN^- anions. Diffuse reflectance spectra reveal that the optical band gap increases by the bromide substitution, which is supported by the DFT calculations. Curiously, bromine-rich compounds where x ≥ 0.8 are light sensitive, leading to partial decomposition after ∼24 h. This study demonstrates that the layered perovskite (MA)₂PbI₂(SCN)₂ tolerates a wide range of bromide substitution toward tuning the band gap energy.

Layered Perovskite (CH3NH3)2Pb(SCN)2I2 Single Crystals: Phase Transition and Moisture Stability

To study stability issues of three-dimensional perovskites, there is a strategy to introduce the thiocyanate ion (SCN^-) into CH₃NH₃PbI₃ (MAPbI₃) to solve these problems. Here, we report the bulk growth of layered perovskite MA₂Pb(SCN)₂I₂ single crystals by different growth methods in an ambient atmosphere. We also investigate the structural determination and refinements, band gap, and photoluminescence of MA₂Pb(SCN)₂I₂ single crystals. More importantly, the phase transition and stability of MA₂Pb(SCN)₂I₂ are systematically demonstrated. MA₂Pb(SCN)₂I₂ undergo the reversible single-crystal to single-crystal phase transition in the orthorhombic systems from the space group Pmmn (no. 59) to the space group Pmn2₁ (no. 31) at low temperature. Moreover, the temperature-dependent recovery behaviors of MA₂Pb(SCN)₂I₂ single crystals, powders, and thin films at high temperature are studied in detail. Besides, the moisture stability of MA₂Pb(SCN)₂I₂ is described when exposed to moisture condition by the experiment and theoretical calculations. It would be interesting to not only conduct a comprehensive study on the crystal structures and the phase transition processes of layered perovskites but also provide guidance for further optoelectronic applications of these perovskite materials.

Two-Dimensional Cs2Pb(SCN)2Br2-Based Photomemory Devices Showing a Photoinduced Recovery Behavior and an Unusual Fully Optically Driven Memory Behavior

The rapid development of Internet of Things and big data has made the conventional storage devices face the need of reforming. Rather than using electrical pulses to store data in one of two states, photomemory exploiting optical stimulation to store light information emerges as a revolutionary candidate for the optoelectronic community. However, fully optically driven photomemory with fast data transmission speed and outstanding energy saving capability suffers from less exploration. Herein, a transistor-type photomemory using a 2D Cs₂Pb(SCN)₂Br₂/polymer hybrid floating gate is explored and three host polymers, polystyrene, poly(4-vinylphenol), and poly(vinylpyrrolidone) (PVP), are investigated to understand the relationship between polymer matrix selection and memory performance. All devices show a photoinduced recovery memory behavior but with two distinctly different photomemory behaviors. In addition to the demonstration of a regular nonvolatile photomemory showing a high on/off ratio of >10⁶ over 10⁴ s, an unusual fully optically driven memory behavior is intriguingly accomplished in the Cs₂Pb(SCN)₂Br₂/PVP photomemory. Using white light as the driver of programming and a blue laser as the main performer of erasing, this device can be switched between two distinguishable states and possesses acceptable data discriminability, as evidenced by its fully optically driven writing (programing)-reading-erasing-reading switching function that shows an on/off current ratio of 10³. This study not only presents the first 2D perovskite-based photomemory but also shows a novel fully optically driven memory that has been rarely reported in the literature.

Development of Hybrid Pseudohalide Tin Perovskites for Highly Stable Carbon-Electrode Solar Cells

Tin-based perovskites degrade rapidly upon interaction with water and oxygen in air because Sn-I bonds are weak. To address this issue, we developed novel tin perovskites, FASnI_(3-x)(SCN)_x (x = 0, 1, 2, or 3), by employing a pseudohalide, thiocyanate (SCN^-), as a replacement for halides and as an inhibitor to suppress the Sn²⁺/Sn⁴⁺ oxidation. The structural and electronic properties of pseudohalide tin perovskites in this series were explored with quantum-chemical calculations by employing the plane-wave density functional theory (DFT) method; the corresponding results are consistent with the experimental results. Carbon-based perovskite devices fabricated with tin perovskite FASnI(SCN)₂ showed about a threefold enhancement of the device efficiency (2.4%) relative to that of the best FASnI₃-based device (0.9%), which we attribute to the improved suppression of the formation of Sn⁴⁺, retarded charge recombination, enhanced hydrophobicity, and stronger interactions between Sn and thiocyanate for FASnI(SCN)₂ than those for FASnI₃. After the incorporation of phenylethyleneammonium iodide (PEAI, 10%) and ethylenediammonium diiodide (EDAI₂, 5%) as coadditives, the FASnI(SCN)₂ device gave the best photovoltaic performance with J_SC = 20.17 mA cm^-2, V_OC = 322 mV, fill factor (FF) = 0.574, and overall efficiency of power conversion PCE = 3.7%. Moreover, these pseudohalide-containing devices display negligible photocurrent-voltage hysteresis and great stability in ambient air conditions.